They need to be fast, they need to be cheap, and they need to be reusable -- on a one-day pit stop, no less. That's the word from DARPA, the Defense Department's no-idea-is-too-far-out shop, which has just launched the Experimental Spaceplane (XS-1) program.

The goal of the XS-1 program is to give the U.S. military a flying machine that can soar to suborbital altitudes, at which point an expendable upper stage (or stages) would separate in order to deploy a small satellite into low-earth orbit. So don't think of an XS-1 craft as a space shuttle heading off on long junkets into orbit itself, despite DARPA's concept artwork. Nor would these space planes be ferrying or piloted by astronauts -- they're to be unmanned.

An XS-1 space plane would also be much faster than NASA's just-retired shuttles. DARPA's pitch to potential designers is that the new machine be capable of hypersonic flight -- in this case, a very ambitious Mach 10 or better, at least once during the tryout phase. Earlier this year, the fourth and final flight in the US Air Force's X-51A Waverider program saw the scramjet-powered vehicle hit a peak speed of Mach 5.1 only briefly, in a flight that lasted a total of about six minutes. (In 2004, NASA's X-43A reached a stunning Mach 9.8 -- for all of about 10 seconds.)

Other technical goals of the XS-1 program include flying 10 times in 10 days and launching a representative payload, of roughly 3,000 to 5,000 pounds, into orbit.

And the space plane system has to be cheap, relatively speaking, coming in at less than $5 million per flight, or roughly one-tenth (or better) the cost of existing launch systems. Cost savings would be achieved in part through smaller ground crews and the absence of specialized (read, expensive) infrastructure.

"We want to build off of proven technologies to create a reliable, cost-effective space delivery system with one-day turnaround," Jess Sponable, DARPA program manager heading XS-1, said in a statement. "How it's configured, how it gets up and how it gets back are pretty much all on the table -- we're looking for the most creative yet practical solutions possible."

One such technology that has already served as a proving ground of sorts is the US Air Force's X-37B space plane. Since 2010, the two experimental, unmanned X-37B systems in service have flown a handful of long-term missions, which the Air Force and Boeing have been happy to publicize while providing few actual specifics. It's quite possible that the orbital flights -- the third of which has been under way for about nine months now -- are the equivalent of technical joyrides, but speculation has ranged from spying missions to satellite deployments.

So get cracking on those blueprints, Tom Swift. DARPA wants to meet up one-on-one with potential space plane proposers in early October.